Transponders are used for example in radio frequency identification (RFID) systems. Conventionally, both the energy supply to a transponder in an RFID system and the data interchange between the transponder and a reading device are effected using magnetic or electromagnetic fields. RFID transponders have an electronic circuit and, depending on the frequency domain, an antenna coil (e.g. 13.56 MHz) or an electromagnetic antenna (e.g. 868 MHz). Via the antenna the energy required for operating the transponder can be taken from the field of the reading device, as well as the data transfer carried out.
Up to a certain distance between reading device and transponder, which is also referred to as the energy range, the transponder can take just enough energy for operating its circuit from the field of the reading device. Typical energy ranges of such systems are about 10 cm for ISO 14443 and up to 1 m for ISO 15693-compatible systems.
The range within which communication is possible in the system by data transfer can be increased by using active transponders, that is, transponders with their own energy supply. The energy supply of the active transponder, for example in form of a battery, operates its electronic circuit. Conventional transponders without their own energy supply are referred to as passive transponders, in contrast.
RFID systems, for example for different types of coupling, and a load modulation using a subcarrier in inductively coupled RFID systems are described in particular under chapter 3.2 in “RFID-Handbuch” by Klaus Finkenzeller.
For simple and fast contactless communication between two devices, the companies Philips and Sony have developed an inductive transmission technology which is known as so-called near field communication (NFC). This technology is integrated for example in mobile telephones or PDAs to permit communication between the devices or with a computer. The coupling of the two devices is effected via coils, the carrier frequency being 13.56 MHz as in RFID systems. As described more closely in the NFC standard ECMA 340, NFC systems have an active communication mode and a passive communication mode. In the active mode, two NFC units alternately generate their own RF field as a signal carrier, thus switching back and forth, coordinated with each other, between transmit mode and receive mode as in a classic mobile phone system. In the passive mode, however, the two NFC units must agree on which unit acts as a reading device and generates a field which the other unit can then influence by means of load modulation. Primarily due to the small antenna diameters used, NFC systems only permit small communication ranges of up to 20 cm.
Even with active transponders there is the problem that data transfer from the transponder to the reading device is limited by the signals becoming weaker with increasing distance between reading device and transponder. As a rule, ranges no longer than 1 to 2 m can therefore be obtained even with an active transponder for example in inductively coupled systems which transfer data to the reading device on the principle of load modulation. For RFID systems working on the principle of backscatter, however, communication ranges of 10 m are typically obtained.